The present invention relates to a semiconductor laser device.
Conventionally, a semiconductor laser device as shown in FIG. 8 has been provided. This semiconductor laser device 100 has, on an n-GaAs substrate 101, an n-type clad layer 102 made of n-AlxGa1−xAs (x=0.5 or so), an active layer 103 made of AlxGa1−xAs (x=0.1 or so), a p-type clad layer 105 made of p-AlxGa1−xAs (x=0.5 or so) and having a ridge portion 104, a cap layer 106 placed on the ridge portion 104 and made of p-GaAs, a current constriction layer 110 positioned on widthwise both sides of the ridge portion 104 and on the p-type clad layer 105 and made of n-GaAs, and a p-GaAs contact layer 112. The contact layer 112 is formed with a thickness of about 50 μm, where the thickness from the lower end of the substrate 101 to the active layer 103 and the thickness from the upper end of the contact layer 112 to the active layer 103 are made generally equal to each other and smaller than 60 μm.
This laser device 100 is used as a light source in such an optical pickup of an optical disk system as shown in FIG. 9A. A laser beam emitted from a widthwise central portion of the active layer 103 of the semiconductor laser device is split into a main beam and a sub-beam by a diffraction grating 121, which are then reflected by a half mirror 123 and converged to an optical disk 125 by an objective lens 124. A laser beam reflected by the optical disk 125, passing through the objective lens 124 and through the half mirror 123, is received by an unshown photodetector and thereby converted to a reproduced signal. Part of the laser beam reflected by the optical disk 125, especially the sub-beam is reflected by the half mirror 123, returning through the diffraction grating 121 to the semiconductor laser device 100. This return beam returning to the semiconductor laser device 100 comes incident at positions away from the active layer 103 of the semiconductor laser device by about 60 μm on thicknesswise both sides of the semiconductor laser device. This semiconductor laser device is so made that the thickness from the lower end of the substrate 101 to the active layer 103 and the thickness from the upper end of the contact layer 112 to the active layer 103 become generally equal to each other and smaller than 60 μm. Therefore, of the return beam, only part of the return beam lower than the active layer 103 in FIG. 9A comes incident on an end face portion A of a stem 126 on which the semiconductor laser device 100 is mounted. With this arrangement, the return beam is prevented from coming incident on both the end face portion A of the stem 126 and an end face portion B of the semiconductor laser device, as in an optical pickup including such a semiconductor laser device 200 in which an active layer 203 is formed so as to be decentered toward the thicknesswise lower side as shown in FIG. 9B. Thus, the optical pickup of FIG. 9A is made smaller in return-beam reflection amount than the optical pickup of FIG. 9B, so that read signals derived from the return beam reaching again the optical disk and detected by the photodetector undergo less deterioration in S/N (Signal-to-Noise) ratio.
The semiconductor laser device 100 is manufactured in the following processes. As shown in FIG. 10A, on an n-GaAs substrate 101, an n-type clad layer 102, an active layer 103, a p-type clad layer 105, and a cap layer 106 are stacked one on another by MOCVD (Metal Organic Chemical Vapor Deposition) process at a temperature of 700-750° C. Then, widthwise both side portions of the cap layer 106 as well as widthwise both side specified-depth portions of the p-type clad layer 105 measured from its top surface are removed to form a ridge portion 104, and further an n-GaAs layer 107 is grown on the cap layer 106 and the p-type clad layer 105 by MOCVD process (FIG. 10B). In the n-GaAs layer 107, during its growth by the MOCVD process, n-GaAs is grown into a configuration corresponding to the configuration of the surface that is subjected to growth, so that a protruding portion 108 protruding from widthwise both side portions at a widthwise central portion of the n-GaAs layer 107 positioned above the ridge portion 104 is formed. With a resist mask 109 set on widthwise both sides of this protruding portion 108 of the n-GaAs layer (FIG. 10C), the protruding portion 108 of the n-GaAs layer is removed by etching, and then the resist mask 109 is removed, by which a current constriction layer 110 such as shown in FIG. 10D is formed on widthwise both sides of the ridge portion 104. Subsequently, p-GaAs is grown on the current constriction layer 110 and on the cap layer 106 by slow cooling LPE (Liquid Phase Epitaxial) process, by which a contact layer 112 is formed. Thus, the semiconductor laser device 100 is completed (FIG. 8). During the formation of the contact layer 112 by the slow cooling LPE process, the growth temperature is controlled to a highest of about 800° C. so that a relatively thick contact layer having a thickness of 50 μm is formed reliably.
In another manufacturing method of the semiconductor laser device 100, p-GaAs is grown on the current constriction layer 110 and the cap layer 106 by MOCVD process to form the contact layer 112.
A further method using the slow cooling LPE process for manufacturing the semiconductor laser device is available, the method including the steps of forming a first clad layer, an active layer and a lower-side second clad layer on a semiconductor substrate, forming thereon a current constriction layer having a striped groove, and forming an upper-side second clad layer on this current constriction layer by the slow cooling LPE process. In this semiconductor device manufacturing method, the upper-side second clad layer is formed by filling the groove of the current constriction layer by slow cooling LPE process.
However, in the semiconductor laser device 100, in which the contact layer 112 is formed by slow cooling LPE process, since the highest of growth temperatures for the contact layer 112 is about 800° C., dopants of the n-type clad layer 102 and the p-type clad layer 105 diffuse due to the highest temperature, causing the carrier concentration distribution to change as a problem. Further, dopants of the n-type clad layer 102 and the p-type clad layer 105 diffuse so as to reach the active layer, deteriorating the light-emitting characteristics of the active layer and, as a result, degrading the reliability of the semiconductor laser device as another problem.
In order to suppress the diffusion of the dopants of the n-type clad layer 102 and the p-type clad layer 105, it is conceivable to set the growth temperature of the contact layer 112 to around 700° C. However, with growth temperatures around 700° C., it would be impossible to achieve a growth of the contact layer 112 to a thickness of 50 μm by slow cooling LPE process.
There is a further problem that the contact layer 112 formed by the slow cooling LPE process would result in a carrier concentration that decreases along a direction from lower end toward upper end as viewed in FIG. 8 of the contact layer 112. This is due to the fact that since the slow cooling LPE process causes the growth temperature to lower with an elapse of the growth time, the amount of dopant deposition onto the contact layer 112 decreases with the lowering of the growth temperature. Particularly, at surface portions of the p-GaAs layer grown to form the contact layer 112, carrier concentration decreases considerably as shown in a carrier distribution chart of FIG. 11. In FIG. 11, the horizontal axis represents the thicknesswise distance (μm) from the surface of the p-GaAs layer, and the vertical axis represents the carrier concentration (pcs/cm3). Accordingly, it has been the case that after the growth of p-GaAs, an upper end portion of the p-GaAs growth layer is removed, by which the contact layer 112 is formed. This would result in an increase in the manufacturing process of the semiconductor laser device, which has been a cause of increases in the labor and cost for the manufacture.
There is yet further problem that in the semiconductor laser device 100, in which the contact layer 112 is formed by MOCVD process, since the surfaces of the current constriction layer 110 and the cap layer 106, on which the contact layer 112 is to be grown, have depressions or projections as shown in FIG. 10D, the contact layer 112 formed on the surfaces having these depressions or projections by the MOCVD process is subject to occurrence of strain. FIG. 8 shows a result of observing by photoelastic approach the strain of the semiconductor laser device 100, in which the contact layer 112 has been formed by MOCVD process, in an overlapped view. As shown in FIG. 8, the strain due to the formation of the contact layer 112 on the depressed-and-projecting surfaces by MOCVD process has occurred to not only a widthwise central portion C of the contact layer 112 but also a widthwise central portion D of the n-type clad layer 102 and the substrate 101. The strain that has occurred to the widthwise central portion C of the contact layer 112 and the widthwise central portion D of the n-type clad layer 102 and the substrate 101 would adversely affect the light-emitting region, which is widthwise central portion of the active layer 103, causing a deterioration in the light-emitting characteristics of the semiconductor laser device. This problem is further noticeable with high-output laser devices in which the ridge portion of the p-type clad layer is formed relatively large in height. This is because large height of the ridge portion would increase the depressions and projections of the surfaces of the cap layer and the current constriction layer, which are to be formed on this ridge portion.
Furthermore, in the case where the groove of the current constriction layer is filled with part of the upper-side second clad layer by the slow cooling LPE process, there is a disadvantage as described below. That is, in the slow cooling LPE process, depending on the magnitude of supersaturation Δt of the LPE growth solution, the state in which part of the contact layer is filled into the groove of the current constriction layer varies as shown in FIGS. 12A, 12B and 12C. In FIGS. 12A, 12B and 12C, reference numeral 156 denotes a current constriction layer, and 158 denotes an upper-side second clad layer. The terms, supersaturation Δt of the LPE growth solution, refer to a difference between a saturation temperature at the time when p-type dopants such as GaAs, Al or Mg as a solute are melted into, for example, Ga as a solvent, and a supersaturation temperature at the time when the growth is started on the wafer with the temperature lowered from the saturation temperature. Preferably, the upper-side second clad layer 158 is formed so as to form a planar surface on the groove of the current constriction layer 156 and on the current constriction layer 156 as shown in FIG. 12B. However, the slow cooling LPE process is liable to variations in supersaturation Δt of the LPE growth solution at the plane of the wafer. As a result, growth defects would occur partly within the wafer plane according to the variations in supersaturation Δt as shown in FIGS. 12A and 12C.